Bumper is one of important parts of car. It is a decorative car exterior part. It not only has decorative function, but also has function of absorbing and mitigating external impact, protecting front and rear car bodies, safety of pedestrians. In the past, front and rear bumpers of automobiles were stamped into channel steel from steel plates, which were welded or riveted together with frame longitudinal beams. There was a large gap with body, appearance was not beautiful. With development of automobile industry and application of engineering plastics in automobile industry, automobile bumpers have also undergone innovation as an important safety device. At present, in addition to maintaining original protection function of front and rear bumpers of car, it is also necessary to pursue harmony and unity with overall shape of car body and lightweight.
With application of plastic bumpers, domestic scientific and technical personnel have conducted a lot of research on bumper mold design. Yuan Yingying used MoldFlow software to analyze bumper mold filling process and studied factors affecting bumper warpage deformation; Yu Ling optimized automotive bumper mold gating system based on Mold-Flow software; Hu Xuechuan used mold flow analysis software Mold-Flow to simulate injection molding process of an automobile front bumper, determined number and location of gates to achieve balance and optimization of gating system. These studies have focused on application of mold flow analysis in optimizing process parameters, gate location and quantity. To obtain a qualified bumper, product design, mold design, injection process and other stages are very important, especially mold design stage plays a decisive role.
Now, comprehensively using CADCAE technology to carry out mold design and mold flow analysis for a certain car bumper, difficulties in mold design process have been solved and qualified products have been obtained.

 

(A) Bumper model

 

(B) Partial buckle and undercut structure
Figure 1 Bumper structure
Bumper structure is shown in Figure 1. Appearance quality requirements are high, external dimensions are 1910mm*640mm*781mm, and average thickness is 2.8mm. Product material is TEO+20%Td. Material is filled with low-density minerals, which reduces weight of product while maintaining all properties of traditional high-density product, and has better process performance. In addition to buckle structure, bumper also has undercuts on both sides, as shown in Figure 1(b). Difficulties of bumper injection molding are: ① gating system design; ② mold parting surface design; ③ launch system design; ④ cooling system design.

First, determine location and quantity of gates based on experience, and then verify feasibility of program through mold flow analysis and predict potential risks, so as to optimize design of gate program. In process of bumper mold design, it is important to select number of gates, determine position of gates to control balance of runners, opening and closing time of sequence valve. In the past, design was only based on experience, so many trial molds and repairs were required, which caused mold development cycle to be extended and increased manufacturing cost.

 

(A) Experience program

 

(B) Optimization plan
Figure 2 Comparison of warpage between two pouring schemes
During development of bumper mold, mold flow analysis was used as standard process, and pouring plan shown in Figure 2(a) was determined based on experience. Through simulation analysis, it was found that location of weld line did not meet molding requirements, and warpage of product was relatively large. According to simulation results, gate position is changed, sequence valve is used to control opening and closing time of gate, gating system and cooling system are optimized at the same time, which shortens mold development cycle, avoids potential risks, improves quality of product molding, reduces manufacturing costs. Simulation result of optimized scheme is shown in Figure 2(b). On the one hand, position of weld line is changed by adjusting position of gate, opening and closing time of sequence valve. On the other hand, a reasonable cooling system has improved warpage of product, and warpage has been reduced from 7.4mm to 5.8mm.

 

Figure 3 Pouring scheme
According to injection simulation analysis and optimization, pouring plan of bumper mold is shown in Figure 3, with 10 pouring points set up, and a pouring system that uses a hot runner sequence valve to convert to a common runner. Gate dimensions are as follows: ①Gate 1 is a fan-shaped lap gate, with a starting size of 20mm*5mm and an end size of 100mm*1.8mm; ②Gates 2, 3, 4, 5, and 6 are rectangular lap gates. Starting size is 20mm*5mm, and end size is 20mm*1.8mm; ③Gates 7, 8, 9, 10 are fan-shaped lap gates, starting size is 12mm*10mm, and end size is 20mm*1.8mm. Common runner size (U-shape) between hot runner and cavity is 12mm*10mm. Size of hot runner (circular) is ϕ22mm. Opening sequence of sequence valve is gate 1, 2 → gate 5, 6 → gate 3, 4 → gate 7, 8 → gate 9, 10.

 

(A) Exotype

 

(B) Internal typing
Figure 4 Mold parting
There are mainly two types of bumper mold design, namely outer parting and inner parting, as shown in Figure 4. Outer parting is a traditional parting method. Parting surface is the largest contour line of product in main drawing direction. Mold structure and ejection mechanism are simple. Molding cycle is shorter than inner parting. Disadvantage is that there is a greater risk of parting lines on the surface of product, polishing process needs to be added during spraying, which increases labor costs, bumper has high appearance quality requirements, so it is not used.
Internal parting can avoid problem of parting line, because parting surface of internal parting is on non-appearance surface and is set on inner side of rounded corners around bumper to avoid defects on the surface of product. In order to realize internal parting of mold, mold structure uses secondary track change control technology to inject bumper at one time to ensure appearance quality of bumper, reduce processing procedures, and save processing cost. However, technology is more difficult and technically risky. Cost of mold is higher than that of outer parting mold. However, bumper produced has a beautiful appearance and is widely used in middle and high-end automobile bumper molds. In order to ensure appearance quality of bumper, considering mold structure, price and cost, internal parting technology is used for mold design.

 

Figure 5 Three-level launch organization

 

Figure 6 Second level launch

 

Figure 7 Inclined push rod mechanism

 

Figure 8 Third level launched
According to different parting surface, method of pushing out bumper mold is also different. Molds with external parting are generally pushed out at one time, either mechanically or hydraulically. Mold ejection system using internal parting is more complicated and requires multiple ejections. Mechanical ejection cannot meet requirements, hydraulic cylinders and nitrogen springs are required. Hydraulic ejection process is smooth, has less impact on mold parts, has a long molding cycle and has risk of oil leakage, while nitrogen spring is introduced quickly, has a large impact on mold parts, requires high strength of mold parts, and molding cycle is shorter than that of hydraulic injection. Bumper mold is formed with a three-stage push-out mechanism, as shown in Figure 5. The first stage pushes out 40mm, which is synchronized with mold opening. Product is ejected from fixed mold by hydraulic cylinder piston rod. Second stage pushes out 160mm (see Figure 6). After mold opening is completed, it is pushed out by ejector rod of injection molding machine, two ends are reversed and separated from oblique push rod. As shown in Figure 7, product is ejected from movable mold; third stage pushes out 85mm, using hydraulic cylinder piston rod to push out. Oblique push rod and push rod realize product release from big push block (see Figure 7 and Figure 8), finally robot takes out part.

 

Figure 9 Reverse release mechanism
Structure of bumper mold is complicated, it is necessary to precisely control pushed top dead center and reset bottom dead center. There are many buckles on the bumper, so it is necessary to adopt an oblique push rod mechanism to realize demoulding. Structure design is shown in Figure 7. Another difficulty in launching is reverse release. Diagonal push rod and pull block are used to achieve reverse release. As shown in Figure 9, the first stage of mold is 40mm, diagonal push block is retracted to make room for deformation, pull block pulls product out of mold (elastic deformation).

In bumper injection molding, mold temperature directly affects molding quality (deformation, dimensional accuracy, mechanical properties and surface quality) and production efficiency of product, so temperature adjustment system needs to be designed according to material properties and molding process requirements. Temperature adjustment system of bumper mold includes a hot runner heating system and a cooling system, and water cooling is sufficient. Hot runner heating system is provided by hot runner enterprise, and cooling system design is now mainly considered.

 

(A) Fixed mold cooling water circuit

 

(B) Moving mold cooling water circuit
Figure 10 Mould cooling water circuit arrangement
If bumper is not uniformly cooled, it will easily cause warpage and deformation, resulting in quality defects. In addition to following general principles of mold cooling system design, design of bumper mold waterway also has following design features: ①Design conformal waterway along shape of product, make distance between all cooling pipes and cavity surface as equal as possible to ensure uniform cooling. To reduce warpage and deformation of product, cooling water circuit is shown in Figure 10; ②Diameter of water hole is ϕ15mm and diameter of water well is ϕ24mmn, which ensures sufficient heat transfer area; ③Distance between waterways is 60mm, distance between waterways is 25mm, 15mm away from other non-adhesive planes; ④Mold adopts a centralized water supply method, and water collection block is designed to connect with injection molding machine; ⑤Length of a single cooling circuit is guaranteed to be within 3m, temperature difference between inlet and outlet cooling water is within 2℃; In order to reduce deformation of bumper, dynamic and fixed mold temperatures are independently controlled, dynamic mold temperature is 5°C higher than fixed mold temperature.

 

Figure 11 Actual bumper
After mold is manufactured, mold is tested on HTF3300 injection molding machine, material grade is TYC1152XC2, and injection molding process parameters are set as barrel temperature 230℃, injection pressure 11MPa, holding pressure 7MPa, and molding cycle 55s. Weight of trial-produced product is 3.61kg, which meets product requirements. Molded bumper is shown in Figure 11.
(1) Use of computer-aided design technology and computer simulation technology shortens bumper mold development cycle, reduces development costs, and ensures quality of product molding.
(2) In bumper mold design, choice of gate scheme has a greater influence on deformation of product. Practice has proved that it is a quick and effective method to determine gate scheme through injection molding simulation analysis.
(3) Although conformal cooling water circuit increases processing difficulty, it improves heat dissipation efficiency of bumper mold cooling system, shortens molding cycle, ensures uniform cooling of product, and also ensures molding quality of product.
(4) Multi-stage push-out system is adopted to realize demolding of bumper. Multi-stage push-out system is final guarantee for quality of bumper.